Devices for lighting with a black background are used especially in the field of ultra-microscopy, and are adapted to transmit toward an observer only the light transmitted or diffused by the object to be observed which appears brilliant on a black background, in a way similar to that of microscopic dust particles floating in still air and reflecting a sun beam so as to be clearly visible to the naked eye. These devices comprise a screen adapted to arrest all the light rays which might penetrate directly the objective of an optical system, whereby the light beams lighting the preparation or object to be examined are extremely oblique. These light beams are inclined under an angle greater than the corner angle of the objective, which latter thus only receives the light rays diffused or diffracted by the contours of the object, whereby said contours appear lighted on a black background. The black-background lighting devices generally use spherical mirrors for reflecting the light rays.
This lighting mode is used especially with a view to observing small organisms which are difficult to color and the refraction index of which is rather close to that of the medium environing said organisms. Said lighting mode is used more particularly with a view to detecting the presence of certain microbes. These organisms can already be detected when ordinary lighting is used, however, they are become more easily visible on a black background.
One novel and special example of the various applications of black-background lighting method is the detecting or reading of patterns engraved on the surface of spectacle lenses. It should be recalled here that spectacle lenses may be provided with various very lightly engraved patterns located in an area in the vicinity of the external peripheric third of the lens, such patterns representing:
two circles disposed symmetrically on either side of the center, for determining the optical axis of the lenses with a view to allowing the latter to be centered in the associated spectacle frame;
the trade mark or name of the manufacturer;
the optical power of the lens, expressed in dioptries, or--in the case of multifocal lenses--the differential optical power, expressed in dioptries, between the zone of close vision and the zone of remote vision.
Such markings which must not constitute a nuisance for the user of the spectacles are only clearly visible and readable under conditions of "grazing" light ray incidence, and the opticians who look for these markings so as to get knowledge of the characteristics of a lens or to effect its centering in a spectacle frame encounter considerable difficulty when endeavouring to read such markings while using a laterally lighting lamp. In certain occurrencens the reading of these markings is still more critical, especially when after an accidental destruction of the lenses, only fragments of reduced dimensions are available, and when one looks for that fragment (or those fragments) which carries (or carry) the marking of the dammaged spectacle lens.
Thus it has been recognized that there existed a need for an observing device which could be used in a particularly easy manner. This requirement may be met by the known devices used in the field of ultra-microscopy; however these known devices are too expensive and are not well adapted to be easily handled and to allow the observation or examination of spectacle lenses to be easily carried out.